The blending of two or more gases to form a predetermined homogeneous gaseous mixture is fundamental to many industrial processes. As an example, the solar and LCD industries currently rely on the use of dilute dopant gas mixtures for doping semiconductor materials, and the like.
In addition to requiring high purity, product gas mixtures used in the semiconductor industry also require a target composition controlled within a very tight tolerance limit to be suitable for advanced semiconductor manufacturing. Compared to other gas processing industries, acceptable compositional tolerance limits within the semiconductor industry are narrow and are getting narrower. The allowable variation in each gas component in the product gas can be as low as plus or minus 0.1% of the target concentration with the specific tolerance limits being dependent on the active gas component that is being blended.
Generally, dopant gas mixtures used in advanced semiconductor manufacturing are filled into, stored in and delivered using gas cylinders. There are three typical methods used in practice to fill cylinders with gas mixtures, namely pressure-based methods, gravimetric-based methods, and dynamic blending methods. The pressure-based method uses pressure changes in the cylinder as each component is added to determine when the correct amount of each component has been added to the cylinder. The setup for the pressure-based method is very simple and easy to implement. However, the mixing precision of the method is very low because it depends on the accuracy of pressure and temperature measurements on the system that is generally not precise due to heating that occurs as the gases are compressed into the cylinder as well as due to other potential sources of system temperature change not accounted for during pressure change. On the other hand, gravimetric-based methods can produce mixes with tighter tolerance limits and is used widely in the gas industry. The gravimetric-based method involves adding a specific mass of each component to the cylinder using a weigh-scale. Because weight measurements are temperature insensitive, this method provides a more accurate means of component addition than pressure-based methods. One limitation of gravimetric systems is the requirement for the added weight of each component of a mixture to be high enough relative to the absolute error of the weight measurement. This limitation of gravimetric systems means that as mixture concentrations decrease, the precision required by the weigh-scale used to prepare the mixture, becomes higher for a given mix tolerance target. With the development of more advanced electronic devices, the requirements for mix tolerance are becoming more stringent and even the most accurate weigh-scales cannot satisfy the high accuracy required by some semiconductor manufacturers. The limitations present with both the gravimetric and pressure-based methods can be overcome through the use of dynamic blending methods, where instead of adding each component of the mixture to the cylinder sequentially, all components are simultaneously added into the cylinder. The added amount of each component is controlled via flow rate control device, e.g., mass flow controller, restrictive flow orifice, and the like. The concentration of the mix can be monitored using an in-line analyzer and adjusted during the blending process by increasing or decreasing the flow of one or multiple components based on the concentration feedback from the analyzer. Because the mixture is blended first in the blending manifold and then compressed or distributed into all the cylinders simultaneously, the compositional variation of mix used to fill a given set of cylinders is very low.
Although the setup of a dynamic blending system can be complicated and costly, it is an attractive option due to its inherently higher mixing precision and for semiconductor users that consume large quantities of mixtures in various balance gases, which poses a logistics and handling challenge. Furthermore, frequent cylinder changes add process variation, increase customer costs and increase safety risks associated with cylinder changes. The use of dynamic blending systems allow customers to have the units installed at their site and to make the mixtures as needed at the point of use with no need for cylinders. By way of example, US Patent Publication No. 2012-0227816 discloses a dynamic blending system designed to dilute dopant gases with improved mixing precision and accuracy. Although the blending process represents an improvement over other on-site blending processes, US Patent Publication No. 2012-0227816 is limited to re-blending of off-spec gasses that must be recycled and re-blended into product specification/tolerance.
Other operational challenges remain with conventional dynamic blending processes which are currently commercially available. For example, unlike blenders used for cylinder filling that allow for relatively easier control of the concentration due to allowances for venting during startup and shutdown, dynamic blenders used on a customer site to make mixtures as needed, are required to achieve higher mix precision while frequently stopping and starting on an irregular or intermittent basis as customer needs fluctuate. As previously mentioned, acceptable compositional tolerance limits within the semiconductor industry continue to narrow. As a result, even slight deviations from the target concentration may lead to off-spec gas product which requires time-consuming corrective actions and/or generation of waste product that must be vented or discarded by other suitable means in a safe manner.
There is an unmet need for an improved dynamic blending system and process that can successfully produce blended mixtures with higher precision at reduced tolerance limits while reducing the generation of waste.